For describing the background of the invention, particular reference is made to the following publications:    [1] G. Cirmi et al. in “J. Opt. Soc. Am. B” vol. 25, no. 7, p. B62 (2008);    [2] G. M. Rossi et al. in CLEO 2014 conference, San Jose, USA, (Optical Society of America, Washington, D C, 2014), SF1E.3;    [3] S.-W. Huang et al in “Nature Photonics” vol. 5, p. 475 (2011);    [4] 0. D. Mücke et al. in CLEO 2013 conference, San Jose, USA, (Optical Society of America, Washington, D C, 2013);    [5] A. Harth et al. in “Opt. Express” vol. 20, p. 3076 (2012);    [6] 0. Mücke et al. in “Optics Letters” vol. 34, p. 118 (2009);    [7] Hanieh Fattahi et al. in “Optica” vol. 1, p. 45-63 (2014);    [8] US 2014/0139921 A1;    [9] WO 2011/157284 A1;    [10] CN 101764341;    [11] CN 201252335;    [12] CN 201054063;    [13] CN 101320191;    [14] WO 2007/149956 A2; and    [15] A.-L. Calendron in “Opt. Express” vol. 21, p. 26174 (2013).
In strong field physics, there is a need for high-energy light pulses with multi-octave bandwidth, e. g. for initiating events occurring within one optical cycle or for creating as (attoseconds) pulses. It is generally known that such high-energy pulses with multi-octave bandwidth cannot be generated by regular laser gain media, but by optical frequency synthesis techniques and nonlinear pulse broadening (e. g. [3]). Multiple amplifiers operated in different spectral regions and pulse shaping (e. g. [8]) are used for creating broadband spectra, wherein frequency synthesis is flexible in spectral shaping allowing tuning of the pulse shape (e. g. [7]). Because of a broad and tunable amplification bandwidth, optical parametric amplifiers (OPAs) or optical parametric chirped pulse amplifiers (OPCPAs) are typical used in frequency synthesizers. OPAs and OPCPAs require broadband seed pulses and high-energy pump pulses. Due to the direct electric field driven processes e. g. in high-harmonics generation or tip emission processes, carrier envelope phase (CEP) stability of the seed pulses is required. The front-end has thus to be CEP stable before the amplification with follow-on OPAs, typically at kHz repetition rates.
The main methods to achieve CEP stability are either active or passive CEP stabilization techniques. Actively CEP stabilized systems (e. g. [14]) are based on fast feedback loops controlling the intracavity pulse dynamics of a laser oscillator, which will be used as a seed pulse source for further amplification. These systems are very complex and subject to failure and instabilities. In addition, they would be sensitive to timing jitter of the pump lasers in follow on amplification stages.
In passively CEP stabilized systems (e. g. [1], [2], [4], [7], [9]-[13]), the same pulses are used to generate e. g. the CEP-stable seed-pulses for the signal channel and the seed-pulses for the pump pulse channel in OPAs and/or OPCPAs. For example, according to [9], a driver pump source with three outputs is used, the first and second one for generating CEP stable broadband pulses and the third one seeding the pump line for OPA or OPCPA based amplification. For obtaining a CEP-stable seed source, difference frequency generation was employed. The passive systems are more reliable because they do not rely on broadband oscillators and elaborate electronics and they are immune to timing jitter of the oscillator pulses.
A first group of conventional passively CEP stabilized systems is based on either Ti:Sapphire (e. g. [1], [2], or [4]) or fiber based pump technologies, which provide sub-500 fs pulses. Ti:Sapphire pump lasers have an inherent and fundamental limitation on average power due to the high heat load inside the crystals. Even if Ti:Sapphire pump technology sustains high-pulse energy, it is limited in average power because of the high quantum defect causing thermal problems. The current fiber technology provides high average power at the expense of pulse energy due to nonlinear effects inside the fiber gain medium. Due to the above limitations, the conventionally used pump sources have general disadvantages in terms of non-scalability of the achievable spectrum and pump energy.
Another group of conventional passively CEP stabilized systems uses Yb-based driver lasers (e. g. [6] or [7]). According to [6], a system is described, which relies on passive CEP stability, an Yb-based driver laser and white-light continuum generation (WLG) in bulk at 515 nm. The driver of the WLG process is the second harmonic of the Yb-based driver laser, resulting in disadvantages in terms of stability and complexity of the set-up. White-light generated at 515 nm is less stable and requires more complicated setup because of multi-photon absorption. The band-gap of suitable materials is typically 5-7 eV and third harmonic generation can contribute to the WLG process. Usually, for WLG at 515 nm, the material needs to be constantly moved or rotated for avoiding degradation and damage. This movement introduces further instabilities, which are incompatible with CEP stabilization. As a further disadvantage of [6], there is no multi-octave WLG from a CEP stable idler.